X-ray imaging devices operating on the principle of computerized tomography (CT) usually utilize a single X-ray source that moves about a body under examination. The regimen followed can be a combination of linear translational and rotational movement of the source relative to the body, using a pencil or fan beam of X-rays, or pure rotational movement wherein a fan beam of radiation is used. The object of these scan regimens is to cause beams of X-rays in the plane of a slice through the body to pass through each elemental volume of the slice in many different directions, and to measure the intensity of the beams after they pass through the body. As is well known, various reconstruction algorithms are available to operate on data so obtained in order to calculate the absorption distribution of radiation in each elemental volume of the slice, and to provide from the computations a two-dimensional display of the slice in terms of its X-ray absorption characteristic.
By reason of the mechanical movements required to effect a scan, the time required to complete a scan is large as compared to physiological movements of individual structural components of the circulatory and respiratory systems. As a consequence, conventional scanning techniques for imaging a beating heart, for example, result in blurred images because the heart moves through several cycles before the scan is complete. One approach to improve image quality is to synchronize the scan with a physiological parameter obtained from the patient, as, for example, in the case of a heart scan, a particular point in the cycle obtained from an EKG reading. In this way, data are recorded only at the same instant during each cycle of movement. While this improves image quality, the image so obtained is made up of data obtained over many heart cycles, and is not a true instantaneous image.
Another and more promising approach is to utilize a plurality of stationary X-ray sources substantially circularly arranged about a patient. By strobing the X-ray sources sequentially, a rotating fan beam of X-rays is generated. The speed at which the strobing occurs is much faster than the physiological movements under investigation, so that the resulting image is of improved quality because, it is obtained while the organ under consideration is substantially motionless. Such an approach is shown in U.S. Pat. No. 4,129,783 U.S. Pat. No. 4,223,225. In both of these patents, and in the so-called DSR system constructed at the Mayo Medical School and described in Science, Vol. 210, 17 October 1980 (pages 273-280), the X-ray sources are conventional hot-cat node X-ray tubes. The description in the '225 patent cited above indicates the use of tubes of 5 cm in diameter, but the source of tubes of this size is not identified. Furthermore, the description indicates that these tubes are placed at about 10.degree. intervals in a circle about 1 m. in diameter. This arrangement provides for 36 tubes with sufficient space between adjacent tubes to permit detectors to be located therebetween. Because so few tubes are used, the image quality obtained is likely to be marginal. The DSR device, on the other hand, contemplates 28 rotating-anode, heavy-duty X-ray tubes distributed around 162.degree. of arc, and rotatable at 15 RPM to permit more than 28 angles of view to be obtained. The diameter of the circle of reconstruction in this device is about 21 cm. Rotation of the gantry on which the X-ray tubes is mounted allows 112 angular views to be obtained in 1/15 second, thereby reducing artifacts from limited sampling and improving spatial resolution. This mode achieves a wider reconstruction field for objects up to 38 cm. in diameter.
Ideally, a CT scanner which requires no gantry rotation or indexing should have about 500 X-ray sources arranged in a circle in order to achieve high-quality images of the whole body; but 250 sources would probably be sufficient to effect high-quality images over a smaller region, such as the heart. The prior art, as evidenced by the above-described patents and the DSR device, do not approach either of these densities of sources. As a consequence, thought has been given to constructing a CT scanner with cold-cathode X-ray tubes, inasmuch as these tubes should be smaller in size and less expensive to produce than hot-cathode X-ray tubes. Cold-cathode diode tubes, such as shown in U.S. Pat. No. 3,970,884 which use a pointed anode rod and a cathode in form of graphite rings that encircle the point, are relatively small in size, but are not practical in view of the problems with isolating one source from the next when the sources are closely packed, and the difficulties and expenses of switching the high voltages required to obtain repeatability in the X-ray output from burst to burst.
Cold-cathode triodes such as are disclosed in U.S. Pat. No. 3,518,433 and in pages 241-253 of CAPACITOR DISCHARGE ENGINEERING-VOL. III by Frank B. A. Frungel (Academic Press, N.Y., 1976) appear to be more easily controllable, although their size appears to be comparable to conventional hot cathode X-ray tubes. Each of the triode types of X-ray sources utilizes an anode rod and a disk-like cathode. Considering the commercially available Frungel triode, it probably could be repackaged in a scanner such that each source would require from 6-8 cm diameter in order to yield about sources 80 sources in a circle about 1.5 m. in diameter.
This number of sources would still require rotating or indexing the gantry to provide an adequate number of views, but the spectral and spatial stability of such tubes when used in a CT environment remains unknown. Furthermore, the problem of shielding anodes of sources adjacent to the anode of the source that is pulsed to produce X-rays remains to be solved. The latter is a formidable problem unless only that anode/cathode pair whose trigger is strobed has a high voltage applied therebetween. If all of the anodes were maintained at a high voltage, stray electrons from a burst of plasma produced by the pulsed source may impact another anode together with secondary electrons produced by U-V radiation thereby causing unwanted X-ray emissions from anodes other than the desired one.
Another disadvantage of the Frungel type of triode is the relatively large trigger voltage required, typically several tens of kV. This increases the cost and complexity of a CT scanner system employing this type of X-ray source.
Thus, despite a well-known need for a CT scanner with a number of X-ray sources sufficient to eliminate the need for indexing, no successful source or scanner is currently available. It is, therefore, an object of the present invention to provide a new and improved tomographic scanner using cold-cathode flash X-ray sources wherein the above-described deficiencies are overcome or substantially reduced.